In electrochemical devices such as lithium ion batteries and electrical double layer capacitors, there is generally used an electrolyte containing a salt such as lithium salts and ammonium salts dissolved in an organic solvent such as propylene carbonate. However, there remains safety problem associated with volatility of such organic solvents.
It is known that some ammonium salts such as imidazolium salts and pyridinium salts liquefy to molten salt below 100° C., particularly around room temperature, and exhibit high ionic conductivity at relatively low temperatures below 200° C. without water or organic solvent added. These salts have been investigated for application as electrolytes for batteries and others owing to their characteristic involatility. As ionic liquids, many examples of N-substituted imidazolium salts and pyridinium salts are known (see Non-Patent Document 1).
A typical organic solvent electrolyte for electrical double layer capacitors includes an electrolyte containing an ammonium salt such as tetraethylammonium tetrafluoroborate dissolved in a polar solvent such as propylene carbonate. However, the ammonium salts are precipitated at low temperatures, causing a problem of lowering the capacity and conductivity. Accordingly, improving the performances at low temperatures is a critical problem to be solved (see Non-Patent Document 2).
There are examples of applying electrolytes consisting only of ionic liquid to electrical double layer capacitors, but such capacitors have a problem that the ionic liquid solidifies or if not solidify, becomes viscous, significantly decreasing the ionic conductivity at low temperatures below room temperature (see Non-Patent Document 2).
There is reported an example of an electrolyte where an ionic liquid is dissolved in an organic solvent, which shows improved low-temperature performances as compared with conventional electrolytes where an ammonium salt is dissolved in an organic solvent. However, the use of organic solvent poses problems in volatility and safety (see Non-Patent Document 2).
In a phosphoric acid fuel cell, phosphoric acid with a concentration of 85% by weight or more is generally used as an electrolyte. According to Non-Patent Document 3, the melting point of phosphoric acid with 100% purity is about 40° C., while phosphoric acid with a purity of 91.6% by weight or higher completely solidifies at 23.5° C. or lower temperatures. At a purity of 91.6% by weight or lower, solid and liquid co-exist at 20° C. or lower temperatures. Therefore, at an outside air temperature of 20° C. or lower, phosphoric acid solidifies in a phosphoric acid fuel cell out of service, and hence pre-melting of phosphoric acid with heating is required on restarting. Also, on transporting the phosphoric acid fuel cell to an installation site after production, the cell is required to be kept warm with an additional heating device for preventing solidification of phosphoric acid. In addition, solidification of phosphoric acid accompanies with volume change, thereby loading stress to the fuel cell, possibly resulting in degradation. For these reasons, preventing solidification of phosphoric acid is one of the problems of phosphoric acid fuel cells.
Patent Document 1 discloses a method for providing a fuel cell that can prevents complete solidification of the electrolyte during out-of-service period without using heating devices or wetting gas generators. However, phosphoric acid partially solidifies, thereby the resistance of the electrolyte is high on restarting the fuel cell.
Patent Document 2 discloses a method for operating a phosphoric acid fuel cell in which the solidification of phosphoric acid is suppressed on storage of the cell to enable restarting even when the cell temperature lowers to the outside air temperature after the operation is terminated. However, there are still problems of partial solidification of phosphoric acid and decrease in the fuel cell output due to the necessity of lowering operation temperatures.
Patent Document 3 discloses a noticeable method for transporting fuel cells with high reliability and low transporting cost, in which fuel cell degradation caused by solidification of phosphoric acid can be easily and surely avoided. However, phosphoric acid is required to be diluted by supplying and discharging wetting gas or to be re-concentrated by supplying and discharging drying gas. In addition, there remain problems in transportation or termination in extremely cold areas because phosphoric acid is frozen at −17° C. even when diluted to 75% by weight.
Non-Patent Document 4 discloses a normal-temperature molten salt of acid-excess type containing butylamine and phosphoric acid, but gives no specific description on its state and properties at low temperatures.
Patent Document 4 discloses an ionic liquid containing phosphoric acid and a cyclic organic base but does not resolve the above problems, since no description is given on whether the ionic liquid remains liquid or not below 20° C.
On the other hand, since liquid electrolytes are not easy to handle, it is publicly known to impregnate a porous body with a liquid electrolyte to improve the handleability. For example, Patent Documents 4, 5, and 6 disclose electrolyte membranes in which a porous membrane is impregnated with an electrolyte containing phosphoric acid or an electrolyte containing phosphoric acid and an organic solvent not forming a salt with phosphoric acid. However, the problem of low-temperature solidification is still unresolved.    Patent Document 1: Japanese Patent Laid-Open Publication No. H11-238521    Patent Document 2: Japanese Patent Laid-Open Publication No. H6-275297    Patent Document 3: Japanese Patent Laid-Open Publication No. H9-293523    Patent Document 4: Japanese Patent Laid-Open Publication No. 2005-44550    Patent Document 5: Japanese Patent Laid-Open Publication No. H8-88013    Patent Document 6: Japanese Patent Laid-Open Publication No. H8-180891    Non-Patent Document 1: “Ionsei ekitai no kinou sousei to ouyou (Ionic Liquids: The Front and Future of Material Development)” edited by Hiroyuki Ohno, CMC Publishing Co., Ltd., 28-31 (2003)    Non-Patent Document 2: “Functionalization and Application of Ionic Liquids” NTS Inc., 105-123 (2004)    Non-Patent Document 3: J. Am. Chem. Soc., 47, 2165 (1925)    Non-Patent Document 4: “The 35th Symposium on Molten Salt Chemistry” 81-82 (2003).